Effects of food structure and ingredient interactions on antioxidant capacity - Oxidation in Foods and Beverages and Antioxidant Applications - page 325

(Brown et al., 1998; Hider et al., 2001; Psotova et al., 2003). The kinetics and
mechanisms of reactions of polyphenols with ferric ions (El Hajji et al., 2006;
Hynes and O'Coinceanainn, 2004), and the binding constants of complexes have
been reported (Andjelkovic et al., 2006), although the interactions of plant
polyphenols with transition metal ions require further study. Ortho-
dihydroxyphenols are effective at chelating metal ions, and phenolic acids
containing the ortho-dihydroxy (catechol) substitution in an aromatic ring
(caffeic acid, chlorogenic acid, protocatechuic acid, gallic acid, catechol and
methylgallate) were shown to form complexes with ferric ions (ChvÂtalov et
al., 2008).
However, the effectiveness of antioxidants by metal chelation depends on
food structure, with chelating agents being more effective in oils than in
emulsions. This can be due to the complex between an antioxidant and a metal
having a lower stability constant or it may be due to the complex undergoing
ionic reactions in an aqueous medium. It was clearly demonstrated that the
amount of complex formed by chelation of Al(III) by three mono-site ligands: 3-
hydroxyflavone, 5-hydroxyflavone and 3 0 ,4 0 -dihydroxyflavone was reduced by
the presence of water molecules in the medium (Dangleterre et al., 2008).
Metals catalyse decomposition of hydroperoxides, but although metal
chelating agents may prevent this or reduce the rate of decomposition, effects
of polyphenols in aqueous foods may be more complex. Many molecules with
antioxidant activity are also able to reduce metals, and this may have a strong
effect on the oxidative stability of foods (Perron and Brumaghim, 2009). The
reduced form of iron is a much more effective catalyst of lipid oxidation than the
oxidized form. Fe(II) is much more effective at catalysing hydroperoxide
decomposition than Fe(III). Consequently polyphenols that reduce Fe(III) to
Fe(II) may catalyse lipid oxidation rather than retarding it (Paiva-Martins and
Gordon, 2002). This effect occurs mainly in aqueous systems, where electron
transfer reactions are favoured due to the polarity of the medium.
Hydroperoxide stability assessed by the ratio of peroxide value to thio-
barbituric acid reacting substances was higher for oil samples containing the
effective chelating agents citric acid or chlorogenic acid than for -tocopherol in
the presence of added ferric chloride (Maisuthisakul et al., 2006). However, the
ratio of peroxide value to thiobarbituric acid reacting substances for the samples
containing antioxidants fell rapidly to lower values in a soybean oil-in-water
emulsion than in the soybean oil. This was due to increased hydroperoxide
decomposition in the emulsion at the same peroxide value. The complex formed
between caffeic acid and Fe(III) has been shown to decompose by an electron
transfer reaction in aqueous solution (Hynes and O'Coinceanainn, 2001).
13.4.2 Interactions with proteins
Enzymes may remove antioxidants from food systems. This occurs particularly
in plant tissues, where the action of the enzyme polyphenoloxidase may remove
phenolic components by enzymic browning. The reaction is usually associated